On 09/06/10 21:36, Jacob Keller wrote:
Dear Crystallographers,
does anyone know of any conceptual reason why a reverse translatase enzyme
(protein-->nucleic acid) could not exist? I can think of so many things for
which such an enzyme would be helpful, both to cells and to scientists...!
Unless there is something I am missing, it would seem to me conceptually
almost impossible that it *not* exist.
See: "The RNA/Protein Symmetry Hypothesis: Experimental Support for Reverse
Translation of Primitive Proteins"
Masayuki Nahimoto, J. Theor. Biol. (2001) 209, pp 181-187.
In which Nahimoto proposes such a system, and additionally proposes that
it actually existed early in the development of life on this planet.
Reasons why it "could not exist" - No. Reasons why it would be very
difficult - yes. And plenty of reasons why Nahimoto is probably wrong
about it having actually existed:
There is absolutely no evidence presented that such a system was ever in
operation in the history of life on this planet.
Current theories such as the RNA World are much more likely explanations
for how life as we currently know it may have developed from a
pre-biotic state.
DNA replication, DNA=>RNA transcription, and RNA=>Protein translation
all depend on nucleic acid base pairing for part of their specificity.
It truly is the secret of life. And it would not be especially helpful
in Protein=>RNA reverse translation.
Forward translation takes place in the ribosome, but extra specificity
is "smuggled in" via a large set of tRNAs and tRNA charging enzymes, in
reactions which took place beforehand, which are then made use of
through the base-pairing codon:anti-codon recognition.
Reverse translation would most definitely not be running forward
translation in reverse;
the specificity cannot be handled ahead of time, it needs to be
available at the site of reverse translation itself when each successive
peptide residue is presented.
Progressivity: If different recognition sites are swapped in, this has
to be done while keeping place in both the protein chain and in the
growing nucleotide chain. Possibly the protein chain might be cleaved
during the process. The chemistry and geometry of peptide residues is
far more variable than that of nucleotide residues.
The genetic code of reverse translation would be completely independent
of that in forward translation. For the two to have matched up (in the
proposed naturally occurring RT system) would have been extremely
fortuitous, imposing a strong barrier to the introduction of such a system.
Difficulty in dealing with post-translational modifications disulfides,
cyclical peptides, acetylation, phosphorylation, etc.
A peptide sequencer coupled with a nucleotide synthesizer accomplishes
somewhat the same thing, but on a macroscopic scale. This is an
impediment to the motivation for constructing a reverse translatase
enzymatic system.
Cheers,
--
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All Things Serve the Beam
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David J. Schuller
modern man in a post-modern world
MacCHESS, Cornell University
schul...@cornell.edu
